The present description relates to a system for manufacturing of integrated sockets in oriented plastic pipes. The described system is included within the field of manufacturing processes of molecularly oriented profiles, in particular within discontinuous systems.
Molecular orientation is a process wherein, by applying a mechanical deformation to a previously extruded pipe or blank, under suitable conditions of temperature, pressure, deformation speed and deformation radius a substantial modification of its mechanical properties is produced, principally the sigma of the material, impact resistance, creep improvement, resistance to crack propagation, Young's module, etc.
In such molecular orientation processes, an pipe highly resistant to bending or deformation is produced, with less raw material and with identical or superior features, thanks to the better resistance to bending or deformation of the material.
There are various methods or systems for the manufacturing of tubular profiles, which can be grouped in two categories: Continuous or line systems and discontinuous systems.
Generally, discontinuous systems are based on producing the molecular orientation “element by element”, which is achieved by an expansion of a blank within a mould which provides the definitive shape of the tubular profile.
Line processes are different and the orientation occurs continuously and simultaneously at the actual extrusion, there being no interruption in the process, nor intermediate stock, or the like.
Oriented pipes can be joined together to form a fluid conduction line with at least two different manners. One approach is by sleeves, which are independent interconnection parts that can be assembled on tubes with a constant, circular section. Another approach is by the formation of a socket in each pipe; the socket makes it possible to insert the end of one pipe into the socket of another pipe. Each pipe has two different ends, one plain and another formed with a socket, which can be assembled into a line, and the socket can have sufficient space to house a gasket seal of flexible material, which ensures hermetic closure of the pipe network.
The continuous or line production processes, due to their nature, may require that the socket-forming process is a process separate from that of the molecular orientation. For this purpose special machines are usually required which make the socket in a later stage performed after the orientation.
EP0700771, U.S. Pat. No. 3,929,958 and U.S. Pat. No. 3,160,130 describe examples of processes which use auxiliary machinery or tools in order to produce sockets in pipes after the orientation phase.
In EP 0700771, a pressurized-fluid workhead is used for permanently bell-forming end portions of plastic pipes. This document describes how the pipes can be associated with special workheads which combine shaped buffers with forming chambers to heat-form the pipe, cooling it inside the workhead in order to render the belling permanent.
U.S. Pat. No. 3,929,958 describes a process for producing radially-expanded socket ends on thermoplastic pipe lengths wherein the heated end of a pipe is conformed with a mould to produce an internal radially expanded peripheral groove.
U.S. Pat. No. 3,160,130 discloses a method and apparatus for manufacturing hollow articles by expansion of elongate hollow rigid blanks, the method comprising deforming the blank starting at one end, or at an area close to one end, and proceeding along its length. The deformation is achieved by the use of forces, attention being paid during the process to the amount, rate and location of the deformation caused by those forces. This document also describes the control of those deformations during the process.
Discontinuous processes permit the socket to be formed integrally, i.e. the socket can be formed at the same time as the pipe is formed, or made similarly to the aforementioned, as a process external to that of pipe orientation.
Various patent documents can be cited for the production of non-integral sockets, as an extra process, the following being representative:
WO 2002/00926 describes a system for heating the oriented pipe with water, wherein the heating zone is limited by two caps so that once heated and under pressure the pipe is formed against a stamp which provides the shape. Subsequently, the blank is cooled and the water is extracted.
EP 0930148 describes a heating machine which heats the zone to form the pipe at a temperature similar to that used for orientation. Subsequently, a sharp expansion element is inserted, which causes the heated material in plastic state to take the form of the profile of the expansion element, and the materials is then cooled below the temperature of the plastic-solid transition. Once cooled, the parts forming the rubber seal are retracted, until the expansion element has a constant sector, and this element exits the interior of the already formed pipe.
Systems for making integrated seals include U.S. Pat. No. 4,340,344, WO 98/56567 and WO 90/15949 (PCT/AU90/00265). In these cases, the described systems comprise, at one end of the mould, a cavity where on expanding the pipe, a part of the pipe expands into this section, giving rise to the socket, and is subsequently cooled by contact of the pipe metal with the mould.